Our experimental setup is presented in Fig. REF c. Particles in our system are self-propelled, elongated bristle robots (Hexbug, Nano). Each robot has 12 flexible legs on which it jumps due to the rotation of an internal, battery-powered motor [1]}. The motion of a single bristle-robot can be described as directed random motion with a persistence length of more than 1 m [2]} (larger than the radius of our system, which is less than \(0.2\)  m).
Collections of bristle-robots exhibit complex behavior, including the formation of giant clusters [1]}, [4]}, [5]}. We place the bristle robots in a half-circular arena with rounded edges to prevent particles from getting stuck in corners.
The arena is equipped with a detachable partition which can be inserted at will and rotates freely on an axle. The experiments are recorded by a camera (Logitech, Brio). Half of the videos were mirrored to account for a slight chirality of particles making the system asymmetric around its center. However, it was verified that this asymmetry does not affect forces applied by particles from one side or the other beyond the range of statistical error, and specifically that the mean force they apply on walls is monotone in the density of particles, regardless of the side of the system in which they are at (see Supplemental Material). Instead of mirroring, operating the machine in a protocol where the wall is not placed in the center can also be considered. However, to clarify, simplify calculations, and make the comparison to an ideal gas more straightforward, we chose to mirror half of the videos and operate the information engine according to the previously described protocol.
